Innocuous, age-sensitive assays of immune and muscle function have been developed and tested in cross-sectional studies of mice. These assays provide valid measures of function and can be performed without harm to the mice. The overall goal is to develop reproducible and valid biomarkers of aging. Our working hypothesis is that the proposed assays of immune and muscle function can distinguish mice that are aging at different rates. The hypothesis will be supported if changes in immune and muscle function are highly correlated, and if mice that exhibit early declines in immune and muscle function die at an early age. Two strains of hybrid mice (B6D2F1 and B6C3F1) will be used to mitigate the possibility of strain-specific artifact. Half of the mice will be fed ad libitum and half a calorie restricted diet that retards the aging rate. The principal design is longitudinal, in which 30 male mice in each of the four groups (two strains x two diets) will be tested with assays of immune and muscle function at 6 month intervals until near the end of their lifespan. This design will provide data on relations among putative biomarkers and between the biomarkers and lifespan and pathologic processes. A separate group of mice will be tested at 6 month intervals with the innocuous assays and then be sacrificed at 21 months of age (ad libitum fed mice) or 30 months (calorie restricted mice). This cross-sectional component will provide material for tests of immune and muscle, function that require access to internal tissues, permit comparisons between the innocuous assays and more traditional measures, and identify biochemical correlates of functional changes. The data will be analyzed by an iterative, nonlinear partial least squares approach which has proven very useful for the study of longitudinal data in early life but has not yet been exploited for studies of aging. Each of the assays can be applied with only minor modifications to human beings. Consequently, several of the assays should provide useful biomarkers of age-specific physiologic decline suitable for interventional studies in human beings as well as for experimental investigations of rodents.